1. Field of the Invention
This invention relates to an image pick-up apparatus for converting an optical image into electrical signals, and particularly to an image pick-up apparatus having an exposure control device for automatically controlling the amount of an exposure.
2. Description of the Prior Art
Hitherto, for movie photography, use has been made of so-called auto-irises to detect the level of the video signal obtained by the system's image pick-up arrangement for controlling the aperture value of a diaphragm. Because an output signal from the image pick-up arrangement is used, such a system can control exposures with a high level of accuracy.
Recently, technological advances in semi-conductors and progress in magnetic recording technology have resulted in development of solid-state image pick-up elements such as CCD's, BBD's, etc., and in proposals for image pick-up apparatuses for still pictures. However, during still photography, if an exposure control is not accomplished quickly, a photographer will miss the opportunity for shooting pictures. Thus, it is desirable to perform continuous auto-iris operations. This increases the system's power consumption and decreases portability. To overcome these problems, it may be possible to use an arrangement with a power switch separate from a release switch. However, this lengthens the time from the closing of the power switch until stabilization of an image pick-up element and a signal processing circuit for securing an appropriate aperture value. It also increases the length of time required to drive a diaphragm to its appropriate aperture value when the original aperture value deviates significantly from the ultimate value. On the other hand, external light metering elements, such as SPC's, etc., are used for light metering in conventional cameras using silver salt films. This permits a simple arrangement to accomplish a rapid control. However, such a system is disadvantageous with respect to exposure control accuracy when compared with a system using video output signals of an image pick-up arrangement.
Such apparatuses have conventionally been known as having an optical viewfinder for obtaining a clear clean viewfinder image in an image pick-up apparatus for a video camera etc. However, the viewfinder's optical element must be placed to the rear of the diaphragm in order to allow interchange of lenses in such image pick-up apparatuses.
This is so because it is difficult to exercise aperture control accurately unless the position of the diaphragm is in the middle of an image pick-up optical system. Furthermore, if the viewfinder's optical element is provided in front of a diaphragm, mounting an interchangeable lens on a video camera requires the image pick-up optical axis in the interchangeable lens and the optical axis in the viewfinder to match each optical axis in the video camera precisely. This is very difficult to achieve.
This problem can be solved by locating the viewfinder in the rear of the diaphragm. However, in that case, a photographer will be viewing the object through a viewfinder with light passing through an actually stopped down diaphragm. This may darken the field of vision considerably. Hence, this arrangement is disadvantageous for composing the picture and for range finding. Opening the diaphragm fully before picking up an image and stopping it down at the start of image pick-up can go far toward eliminating this difficulty.
However, the dynamic ranges of image pick-up devices such as image pick-up tubes or CCD's, etc., are such that they often cannot function adequately with light passing through a fully opened diaphragm. Hence, light sensing with an ordinary pick-up device in an auto-iris arrangement or such becomes almost impossible.